The present invention concerns improvements introduced in the heat treatment of the body of a preform or of an intermediate-stage container made of a thermoplastic material, this treatment being carried out during the process of manufacture of a container, such as a flask or bottle, in particular by blowing or stretching-blowing, during which heat treatment the preform, while being made to rotate on itself, travels through a heating furnace.
There exist manufacturing processes involving a single blowing operation. Starting with an injection-produced preform, these processes consist in heating this preform and then injecting it and, potentially stretching it, so as to produce the final container.
There also exist processes which entail several distinct blowing and/or stretching/blowing operations. Beginning with an injected preform, these operations consist in effecting a first blowing operation to produce an intermediate container, which is, in turn, reheated and blown once again to produce the final container. A process of this kind and the various steps involved therein is described in Patents Nos. EP-A-0 442 836 and U.S. Pat. No. 5,229,042, held by the Applicant. To introduce some precision into the discussion, FIG. 1, attached, illustrates the diagrammatic shapes obtained during a process comprising two successive blowing and/or stretching-blowing operations and carried out under the conditions indicated in the aforementioned documents: i.e., the initial preform 1 (which is molded or injected) is transformed by a first blowing and/or stretching-blowing operation into a longitudinally- or transversely-oversized container 1.sub.1, which then undergoes heat treatment which, by virtue of the release of stresses, produces an intermediate, contracted container 1.sub.2. After heating, the latter is then, in turn, subjected to a second blowing or stretching-blowing operation, at the end of which the final container 1.sub.3 is obtained.
As illustrated in the attached drawings, with respect to a container preform 1, whether molded or injected, comprising a substantially cylindrical thick-walled body generated by rotation and, at one end, a hemispherical thick-walled bottom 3 and, at the other, a neck 4 incorporating its final shape and dimensions, the aforementioned preliminary heating operation consists in heating the body alone of the preform (excluding the neck) to a temperature greater than the vitreous transition temperature Tg of the thermoplastic material making up the preform.
In practice, heating is carried out by causing the preforms to travel in an oven 5 comprising at least one lateral heating wall (incorporating, for example, heating tubes 6 attached to the oven wall), while at the same time causing the preforms to rotate on themselves (arrow 7) so as to make the temperature within the material uniform. Protective means, i.e., shields 8 in the form of parallel tracks, ensure heat protection of the neck 4. A reflecting panel 9 may preferably be positioned opposite the tubes 6 to reflect back toward the bodies 2 of the preforms the fraction of the thermal radiation passing between two successive bodies.
When containers are produced having bodies that are approximately cylindrical and generated by rotation, the subsequent blowing or stretching-blowing process, which is performed in a mold on a heated preform or on the intermediate container, causes substantially uniform stretching of the heated, softened thermoplastic material in all radial directions (determined in relation to the preform axis). In this case, a container body possessing an even, uniform structure and a lateral wall having substantially constant thickness is produced.
However, problems arise when attempts are made to deviate from this conventional process.
First, difficulties are encountered when the final container is not approximately cylindrical and generated by rotation: for example, containers having a polygonal transverse section, whether triangular, quadrilateral, or pentagonal in particular, in which each approximately plane face extends substantially away from a cylindrical surface surrounding the edges, or containers having flattened bodies (e.g., bottles holding detergent products). In this case, as illustrated in FIG. 2B with respect to a container whose body 10 has an approximately square section, the material of the parts of the wall of the final container the farthest away from the axis A of the initial preform 1 (i.e., the angled parts 11) is subjected to a much higher degree of stretching than that applied to the material making up the parts of the wall remaining closest to said axis (i.e., the central parts 12 of the walls). Because even heating imparts to the material the same stretching characteristics in no matter what parts of the preform body, the greater degree of stretching of the parts of the walls 11 which are ultimately the most distant from the axis A of body of the preform 1 is accompanied by increased thinning of this part of the wall. Accordingly, the finished product is a container having non-uniform wall thickness, which is thinner (thickness e) in the parts 11 most distant from the axis A of the initial preform body (i.e., angled areas in a prismatic body and areas of slight curvature in bodies having an oval or elliptical section) and thicker (thickness E) in the parts 12 closest to the axis A of the beginning preform.
The mechanical strength of a container body incorporating an uneven configuration is not satisfactory.
In addition, the thinnest portions of the walls must possess sufficient mechanical strength, and the thickness of these portions of the wall must remain greater than a predetermined minimum value. Accordingly, in the thickest part of the wall, the wall thickness is excessive, and the quantity of material present exceeds the quantity required for the desired mechanical strength. Raw material is thus wasted.
Moreover, in containers having a flattened body, a deformation, termed "panelling" in the industry, appears on the parts of the wall remaining closest to the preform axis. This deformation results from the fact that the thickness in this part of the wall is excessively high and that the material exhibits a "memory" phenomenon caused by the non-uniformity of the stretching. The excessive thickness is caused by the fact that, during the blowing operation, the preform first comes into contact with the wall of the mold closest to the preform axis.
The same problem reoccurs on a container bottom having a complex shape (e.g., a pronounced oval shape). On this bottom, the thicknesses of the area in which the container is in contact with a plane and which are the most distant from the center are much smaller than those closest to the center.
Finally, all of these problems are exacerbated when the final container, produced from a preform exhibiting complete radial symmetry, is quasi-asymmetrical (e.g., a neck significantly off-center in relation to a prismatic or flattened body and possibly incorporating a handle, for example).
In addition, other difficulties are encountered when attempts are made to strengthen the rigidity of the container body by incorporating therein one or several annular reinforcement zones. According to one known solution carried out for this purpose, one or several annular zones of the body are crystallized. Although the crystallized material does in fact possess increased stiffness, this solution is not thought to be effective in practice, if only because of the coloration exhibited by the crystallized material, which imparts an unattractive appearance to the container.
The aforementioned problems appear with equal frequency in processes involving one or several blowing operations.
In fact, when using a process consisting of several blowing operations, the intermediate container exists in a relation to the initial preform which is similar to the relation of the final container to the initial preform produced when implementing a process consisting of a single blowing operation. Accordingly, the intermediate container produced during implementation of a process entailing multiple blowing operations may, in fact, be compared to a special preform obtained not by injection, but by extrusion of a primary injected preform.
Thus, in the remainder of the text, the term "preform" may be held to designate both (1) the initial preform obtained by injection and used in processes consisting of a single blowing operation and yielding the final container directly or in processes consisting of multiple blowing operations leading to the intermediate container; and (2) the intermediate container itself when a process comprising several blowing operations is implemented.
The problems mentioned above are such that they have, to date, slowed development of the manufacture of containers incorporating complex shapes by means of blowing or stretching-blowing techniques applied to a thick-walled preform, in particular one having a cylindrical body generated by rotation.